新的電腦模擬方法可以讓我們更加瞭解大型逆衝區地震
2011年，東日本大地震襲擊了日本沿岸地區，造成十分嚴重的災情。而數年前地震發源地點附近的地殼就已經開始騷動不安。德州大學奧斯汀分校的研究人員運用電腦模型，探討東日本大地震和發源地點附近偵測到的長微震是否有所關聯。
2011年3月11日，東日本大地震及其造成的海嘯襲擊了日本沿海地區，造成超過15000人喪生。以德州大學奧斯汀分校為首的研究正在開發的電腦模型，可以讓我們更加瞭解是什麼樣的力量造就了這類破壞力極強的地震。圖片來源：Douglas Sprott
大型逆衝區地震(megathrust earthquake)是世上破壞力最強的一種地震。這項研究於2018年12月15日發表在《地球與行星科學通訊》(Earth and Planetary
Science Letters)，結果可以讓科學家更加瞭解驅動這類地震的力量，並使地震災害的評估更加精準。
主要作者Thorsten
Becker是德州大學奧斯汀分校地質科學院的教授，以及德州大學地球物理研究所的研究人員。他說這項研究首度詳盡呈現出東日本逆衝地震之前，相當難以察覺的長微震(tremor)活動出現了什麼樣的變化。
「最後產生破裂的地點附近，有一部份的地殼應力狀態在事件發生兩年前已經發生了變化。」Becker表示，「我們研究得出的這些結果可以補足其他有關地殼變形的研究，並讓我們更加瞭解引發地震的力量。」
德州大學地球物理研究所是隸屬於該校地質科學院的研究單位。
雖然這些長微震的地點讓人好奇它們跟地震之間是否有潛在關聯，但Becker表示現在還無法確定兩個事件之間的關係。不過，長微震的震波訊號有助於改良電腦模型來解開它們的關係。科學家運用新的模擬技術可以創造出四維圖像呈現出地殼以及板塊間的交互作用，進而得出推動斷層的力量隨著時間如何變化。
將震波數據輸入模型之後，模型會統合它和地震前後數年的板塊變形觀測資料，使科學家可以推斷在板塊邊界產生了什麼樣的作用力。在板塊邊界，板塊會下潛至地球內部熾熱且黏稠的地函當中。由於地函處於半融化的狀態，使得堅硬的岩石在此會像軟泥一樣而有特殊的行為模式。瞭解地函的動力學，可以幫助科學家找出作用於斷層之上的壓力在大地震前後的關係。
這項新研究的重要之處在於運用的模型原本是從另一組截然不同的數據發展而來――用來描述地球形狀的大地測量資訊。科學家從地震波跟地球形狀變化這兩組不同的數據中得到類似的結果，使他們對地震模型的準確度有更多信心。
Becker相信正確的研究方向與充足的經費，可以讓科學家運用更加先進的電腦模型研究地震的物理原理，或許還能對地震預警有所貢獻。
目前科學家能做到的極限是提供災害潛勢圖――這種地圖標明出已知的地震帶以及未來數十年地震發生的概率。可以得到更多大型逆衝區地震可能襲擊的時間地點資訊，即使只是未來數年之內的情報，就目前的地震預警來說也是相當長足的進展，並讓政府跟產業有足夠的時間做出因應。
為了達到上述目標，作者希望這篇研究可以對目的為提升地震災害評估的國際計畫有所貢獻，像是由德州大學領導的聯合研究網：隱沒帶模擬合作實驗室(經費來源為美國國家科學基金會)。
此篇研究的經費來自美國國家科學基金會以及日本文部科學省。
New computer modeling approach could
improve understanding of megathrust earthquakes
Years before the devastating Tohoku earthquake struck the coast
of Japan in 2011, the Earth's crust near the site of the quake was starting to
stir. Researchers at The University of Texas at Austin are using computer
models to investigate if tiny tremors detected near this site could be
connected to the disaster itself.
The research could help
enhance scientists' understanding of forces driving megathrust earthquakes --
the world's most powerful type of earthquake -- and improve earthquake hazard
assessment. The study was published on Dec.15, 2018, in Earth and Planetary Science
Letters.
Lead author Thorsten
Becker, a professor at the UT Jackson School of Geosciences and researcher at
the University of Texas Institute for Geophysics, said that this was the first
comprehensive study showing changes in barely perceptible tremor activity
before the Tohoku megathrust earthquake.
"The part of the crust
that is close to the place that eventually ruptured changes stress state a
couple of years before the event," said Becker. "By demonstrating this,
our work complements studies of crustal deformation and our understanding of
the forces driving earthquakes."
The Institute for
Geophysics is a research unit of the Jackson School of Geosciences.
While the location of the
tremors raises questions about their potential linkage to the quake, Becker
said that it's unknown at the moment if the two events relate. However, the
seismic signature of the tremors is helping refine a computer model that could
help untangle the connection. This new modeling technique allows scientists to
create a four-dimensional image of the Earth's crust and interactions between
tectonic plates, showing how forces pushing at the fault change over time.
Once the seismic data was
inputted, the model matched observations of how the plate deformed in the years
before and after the earthquake. This allowed the scientists to make inferences
about the kind of forces taking place at the plate boundary, the point where
one plate dives into the Earth's hot, viscous mantle. In this semi-molten
layer, solid rocks ooze and behave in unexpected ways, so understanding the
dynamics of the layer could help identify the connection between pressure along
a fault before and after a major earthquake.
The new research is
significant because the model was originally developed using a different
dataset: geodetic information about the shape of the Earth's surface. By
gaining similar results using different data sets -- seismic waves and changes
in the planet's shape -- scientists can be much more confident about the
accuracy of earthquake models.
Becker believes that with
the right research and support, advanced computer models can be used to study
the physics of earthquakes and perhaps contribute to improved forecasts.
Currently, scientists can
at best offer hazard maps showing known earthquake zones and a vague
probability of an earthquake in the coming decades. Knowing more about when and
where such a quake might strike, even within a few years, would represent a
significant improvement on current earthquake forecasting and perhaps allow
authorities and industry adequate time to prepare for such an event.
To this end, the authors
hope their study will contribute to global efforts to improve earthquake hazard
assessment, such as the Modeling Collaboratory for Subduction RCN, a new UT-led
research collaboration network funded by the National Science Foundation (NSF).
The study was supported by
the NSF and the Japanese Ministry of Education, Culture, Sports, Science and
Technology.
原始論文：Thorsten W.
Becker, Akinori Hashima, Andrew M. Freed, Hiroshi Sato. Stress change
before and after the 2011 M9 Tohoku-oki earthquake. Earth and
Planetary Science Letters, 2018; 504: 174 DOI: 10.1016/j.epsl.2018.09.035
引用自：University of Texas Institute for Geophysics. "New computer
modeling approach could improve understanding of megathrust
earthquakes."
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